Each metal end-fitting, described for example in U.S. Pat. No. 4,656,720, thus has an internal cylindrical housing whose inlet is provided with a flange defining the "inlet plane" of the end-fitting. The creepage distance of the insulator is defined between the "inlet planes" of its two end-fittings.
The insulating coating of the rod must adhere perfectly to the inside face of the flange in order to prevent any trace of dampness reaching the rod which would cause the insulator to deteriorate very rapidly. Further, the inlet to the end-fitting remains a critical region since that is where power arcs are likely to occur which will deteriorate rod anchoring and sealing.